The present invention relates to the processing of high strength near-alpha titanium alloys, such as Ti 6242-Si (eg. Ti-6Al-2Sn-4Zr-2Mo-0.1Si), to improve the level of their mechanical properties, especially, creep strength. The Ti-6Al-2Sn-4Zr-2Mo-Si alloy was developed to fulfill the need for improved elevated-temperature performance titanium alloys, particularly for jet engine service. It is a "super-alpha" type characterized by its high strength and stability at temperatures up to about 1000.degree. F. A "rotating" grade (RG) and a "premium" grade (PG) of 6-2-4-2 alloy as well as a "standard" grade are available in billet, bar, sheet, and plate. The density of the 6-2-4-2 alloy is 0.164 lb/in 3.
The composition and the range or maxiumums (%) are as follows:
______________________________________ Major Elements Impurity Elements ______________________________________ Aluminum 5.5-6.5 Iron 0.25 Tin 1.75-2.25 Carbon 0.10 Zirconium 3.5-45. Oxygen 0.15 Molybdenum 1.75-2.25 Hydrogen 0.0100 (billet) 0.0125 (bar) Silicon 0.1-0.2 0.0150 (sheet) ______________________________________
The 6 percent aluminum addition in the Ti-6Al-2Sn-4Zr-2Mo composition is a potent alpha-phase stabilizer, while the 2 percent molybdenum addition represents only a moderate quantity of this potent beta-phase stabilizer. The tin and zirconium additions are solid-solution strengthening elements that are neutral with respect to phase stabilization. The net effect of this combination of alloying elements is the generation of a weakly beta-stabilized, alpha-beta alloy. Since it is weakly beta-stabilized, the alloy is also properly described as a near-alpha, alpha-beta alloy. This term is frequently referred to in abbreviated form as simply "near alpha" and the Ti-6Al-2Sn-4Zr-2Mo alloy is popularly classified with the "super" alpha compositions.
The Ti-6Al-2Sn-4Zr-2Mo alloy evolved from research conducted to improve upon the properties inherent in high-aluminum-content titanium compositions, principally high strength at elevated temperatures. However, the requirement for high strength at both room temperature and 1000.degree. F. existed along with the need for a composition having greater thermal stability than the high-aluminum-content binary alloys. The additions of molybdenum, tin, and zirconium to the Ti-6Al base gave this new alloy the balance to satisfy these requirements. The beta-stabilizing addition, molybdenum, increases room- and elevated-temperature tensile strength and serves to enhance stability, while the combination of aluminum, tin, and zirconium maintain the long-time (creep) elevated temperature strength. The increase in density resulting from the 8 percent heavier metal additions (tin, zirconium, and molybdenum) is small, while the increase in toughness due to these additions is significant. Since the combination of alloying elements net only a weakly stabilized beta content, the alloy is weldable.
The effect of aluminum in this composition on the allotropic transformation in titanium is to stabilize the alpha phase and increase the beta transus temperature to about 1815.degree. F. Variations in alloy composition and, in particular, variations in oxygen content, affect the beta transus temperature. Oxygen and aluminum are strong alpha-phase stabilizers. Tin, zirconium, and especially molybdenum tend to lower the beta-transus temperature.
The transformation kinetics of Ti-6Al-2Sn-4Zr-2Mo have been studied by conventional quench techniques. The study has placed the Ms temperature at about 1470.degree. F. and the Mf temperature at about 1415.degree. F. Transformation by nucleation and growth is very rapid.
The structures of Ti-6Al-2Sn-4Zr-2Mo alloy are typically massive equi-axed alpha in a transformed beta matrix. The equi-axed alpha grains in sheet product tend to be smaller than is found in forgings and tend to be present in greater proportion than in forgings. Primary alpha is typically about 80 to 90 percent of the structure in sheet and can range somewhat lower than this in a forged product. As in other near-alpha alloys, small amounts of residual beta phase can be observed metallographically within the transformed beta portion of the structure. The occurrence is typically between the acicular alpha grains of the transformed phase.
In addition to the standard constituents of Ti-6Al-2Sn-4Zr-2Mo, a silicon-containing modification of 6-2-4-2 is made; the nominal silicon content is 0.2 percent. Silicon additions to 6-2-4-2 probably result in the precipitation of a silicide dispersed phase and the major mechanical property benefit is to improve the elevated temperature creep strength.
Ti 6242-Si proved to have a very good mechanical characteristics in a high temperature environment compared to other alloys, but it appears to be highly desirable if the creep resistance of near-alpha titanium alloys can be improved, so that some of the heavier nickel-based super alloy components in jet engines can be replaced by titanium alloys. Although extensive work has been carried out to improve both creep strength and stability of these alloys by processing chemical modifications, current commercial near-alpha titanium alloys (e.g. Ti-6Al-2Sn-4Zr-2Mo-0.1 Si) still have limited creep resistance in the 950.degree. F. temperature range.
At the present time, the creep resistance of this alloy has been said to relate to micro structure variations (in the magnitude of optical magnifications), but the correlation between creep and micro structure has not been adequately demonstrated. Furthermore, there exists a broad range of creep properties within a given micro structure type, which has not been satisfactorily explained.
It is, therefore, an outstanding object of the invention to provide a process schedule which improves the creep properties of near-alpha titanium alloy forgings in the 950.degree.-1050.degree. F. temperature range.
Another object of this invention is the provision of a process schedule which eliminates or reduces the incoherent precipitates in near-alpha titanium alloy forgings.
A further object of the present invention is the provision of a process schedule which produces improved creep properties in near-alpha titanium alloy forgings and which produces excellent combinations of tensile and fracture toughness properties.
It is another object of the instant invention to provide a near-alpha titanium alloy forging which can replace heavier nickel-based super alloy components in environments which require high creep resistance at elevated temperatures.
A still further object of the invention is the provision of a forging method for controlling the disolution state of silicon and/or tin precipitates in order to achieve higher creep resistance in Ti-6242-Si alloy.
It is a further object of the invention to provide a forging method which produces finely dispersed precipitates (solute-segregations) which act as effective barriers to immobilize dislocation movement and/or grain boundry migrations, and which thereby reduce creep deformation at high temperatures.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.